Photoreceptors in electrophotography are essentially required to satisfy the following performance properties: (1) they can be charged at an appropriate potential in the dark; (2) dark decay is small; (3) they are capable of rapidly discharging upon light irradiation; and the like.
Inorganic substances which have conventionally been used as photoreceptor materials, such as selenium, cadmium sulfide, zinc oxide, etc., certainly have many merits but, at the same time, involve various demerits. For example, selenium which is widely employed for the time being fully satisfies the above requirements (1) to (3) but needs difficult conditions for the production to the ultimate disadvantage of increased production cost. Further, it is difficult to shape selenium to a belt form due to lack of flexibility, and the selenium photoreceptor requires delicate handling due to high sensitiveness to heat or mechanical shocks. Photoreceptors in which cadmium sulfide or zinc oxide is dispersed in a resin binder have mechanical drawbacks in terms of surface smoothness, hardness, tensile strength, abrasion resistance, and the like so that they cannot be used repeatedly as such.
In an attempt of eliminating the above-described disadvantages of inorganic materials, electrophotographic photoreceptors using various organic materials have been proposed recently, and some of them have been turned to practical use. For example, U.S. Pat. No. 3,484,237 discloses a photoreceptor comprising poly-N-carbazole and 2,4,7-trinitrofluoren-9-one; Japanese Patent Publication No. 25658/73 discloses photoreceptors comprising poly-N-vinylcarbazole sensitized with a pyrylium compound; and Japanese Patent Application (OPI) No. 10785/72 describes a photoreceptor comprising mainly an eutectic complex composed of a dye and a resin (the term "OPI" as used herein means a "published unexamined patent application").
Although these organic electrophotographic photoreceptors have somewhat improved mechanical characteristics and flexibility as compared with the above-described inorganic photoreceptors, they are generally insufficient in photosensitivity and unsuitable for repeated use, and, therefore, do not fully satisfy the desired properties for electrophotographic photoreceptors.
In electrophotographic photoreceptors, photoconduction takes place through (1) a process of generating electric charge upon exposure to light and (2) a process of charge transport.
Electrophotographic photoreceptors wherein both the processes (1) and (2) are effected by the same substance include selenium photoreceptors, and those wherein the processes (1) and (2) are separately carried out by different substances include a combination of amorphous selenium and poly-N-vinylcarbazole. The latter technique in which the processes (1) and (2) are effected by different substances advantageously allows a wide choice in the kind of materials to be used, which leads to improvement of electrophotographic characteristics, such as sensitivity of photoreceptors, acceptable potential, etc. Further, such a large choice of materials means that materials favorable to the formation of photoreceptor coating films can be selected from a wide range.
Many proposals for the above-described separate function type electrophotographic photoreceptors have been made, but a very few of them have been turned to practical use, and those that have been used have had some defects.
Moreover, there has been no theory established up to the present in connection with a combination of a charge generating material and a charge transporting material, and each and every combination is studied making trial and error.